It is known that a block copolymer can be obtained by an anionic copolymerization of a conjugated diene compound and an alkenyl arene compound by using an organic alkali metal initiator. Block copolymers have been produced, see U.S. Pat. No. Re. 27,145, which comprise primarily those having a general structure EQU A--B or A--B--A
wherein the polymer blocks A comprise thermoplastic polymer blocks of alkenyl arenes such as polystyrene, while blocks B are polymer blocks of a selectively hydrogenated conjugated diene. The proportion of the thermoplastic terminal blocks to the center elastomeric polymer block and the relative molecular weights of each of these blocks is balanced to obtain a rubber having unique performance characteristics. In such a rubber having a volume ratio of blocks B to blocks A much greater than 1, the blocks A are thermodynamically incompatible with the blocks B resulting in a rubber consisting of two phases; a continuous elastomeric phase (blocks B) and a basically discontinuous hard, glass-like plastic phase (blocks A) called domains. These domains act as physical crosslinks anchoring the ends of many block copolymer chains.
As A--B block copolymers have only one A block, the strength of such polymers is derived primarily from the inherent entanglements of the various B blocks therein. However, since the A--B--A block copolymers have two A blocks separated by a B block, domain formation results in effectively locking the B blocks and their inherent entanglements in place by the A blocks and forming a network structure. Such a phenomena allows the A--B--A rubber to behave like a conventionally vulcanized rubber that contains dispersed reactive filler particles. These thermoplastic A--B--A rubbers are physically crosslinked by the domains in a network structure as opposed to being chemically crosslinked like a conventionally vulcanized rubber. Since both the network forming (e.g., A--B--A) and non-network forming (e.g., A--B) polymers are thermoplastic in nature, they may be handled in thermoplastic forming equipment and are soluble in a variety of relatively low cost solvents. This is in contrast to polymers which are chemically crosslinked and can not be reversibly processed.
These types of block copolymers are diversified in characteristics, depending on the content of the alkenyl arene compound. When the content of the alkenyl arene compound is small, the produced block copolymer is a so-called thermoplastic rubber. It is a very useful polymer which shows rubber elasticity in the non-vulcanized state and is applicable for various uses such as moldings of shoe sole, etc.; impact modifier for polystyrene resins and engineering thermoplastics; in adhesive and binder formulations; modifications of asphalt; etc. The non-network forming polymers have found particular utility as viscosity index improvers (U.S. Pat. Nos. 3,700,748; 3,763,044; 3,772,196; 3,965,019; and 4,036,910). Non-network forming polymers are also utilized in adhesive and binder formulations and as modifiers or plasticizers for polystyrene resins and engineering thermoplastics.
Network forming block copolymers with a high alkenyl arene compound content, such as more than 70% by weight, provide a resin possessing both excellent impact resistance and transparency, and such a resin is widely used in the field of packaging. Many proposals have been made on processes for the preparation of these types of block copolymers (U.S. Pat. No. 3,639,517).
While in general these block copolymers have a number of outstanding technical advantages, one of their principal limitations lies in their sensitivity to oxidation. This behavior is due to the unsaturation present in the center section comprising the polymeric diene block. Oxidation may be minimized by selectively hydrogenating the copolymer in the diene block, for example, as disclosed in U.S. Pat. No. Re. 27,145. Prior to hydrogenation, the block copolymers have an A--B or A--B--A molecular structure wherein each of the A's is an alkenyl-arene polymer block and B is a conjugated diene polymer block, preferably a butadiene polymer block containing 35-55 mole percent of the condensed butadiene units in a 1,2 configuration.
Both the network forming and non-network forming block copolymers are deficient in many applications in which the retention of properties at elevated temperatures and deformation resistance are required. Non-network forming copolymers are especially deficient in applications in which good mechanical integrity and deformation resistance are required even at room temperatures. In such copolymers, the mechanical integrity of the block copolymers is limited to the integrity of the soft phase (B blocks) since each elastomeric blocks is chemically bound to only one polystyrene domain.
At room temperature, the network forming copolymers are known to have particularly high tensile strengths due to the formation of glassy phase arene block domains which act as physical entanglements within the rubbery B block matrix. The mechanical integrity of these domains appears to control the tensile strengths of these copolymers. At elevated temperatures or in combination with oils, the mechanical integrity of block copolymers are limited to the integrity of the hard phase arene block domains. For example, copolymers having arene blocks of polystyrene have poor mechanical properties at high temperature which may be attributed to the weakening of the polystyrene domains above the polystyrene glass transition temperature (Tg) of 100.degree. C. Improvements in the high temperature characteristics of these block copolymers may be achieved by enhancing the integrity of the alkenyl arene domains to higher temperatures.
These selectively hydrogenated, hydrocarbon A--B--A block copolymers are further deficient in many applications in which interactions are required between it and other materials. Applications in which improvements in adhesion charcteristics may promote improved performance include (1) the toughening of, and dispersion in, polar polymers such as the engineering thermoplastics; (2) the adhesion to high energy substrates in a hydrogenated block copolymer elastomer based high temperature adhesive, sealant or coating; and (3) the use of hydrogenated elastomers in reinforced polymer systems. The placement of functional groups onto the block copolymer may provide interactions not possible with hydrocarbon polymers and, hence, may extend the range of applicability of this material.
Many attempts have been made to improve adhesive characteristics, green strength and other properties by modifying block copolymers with acid compounds, particularly, network forming block copolymers having at least two A blocks and at least one B block. To this end, various methods have been proposed for modifying the polymer with acid moieties, for example, Saito et al. in U.S. Pat. Nos. 4,292,414 and 4,308,353; Hergenrother et al. in U.S. Pat. No. 4,427,828; and Gergen et al. in U.S. Pat. No. 4,578,429. In each case, such modified block copolymers contain functional (acid) moieties only in the diene block. Specifically, Saito et al. and Hergenrother et al. attach anhydride moieties to a partially hydrogenated monovinyl arene/conjugated diene block copolymer by the so-called "ENE" reaction. Gergen et al. describe a block copolymer which is a thermally stable, selectively hydrogenated, high 1,2 content substituted vinyl arene/conjugated diene block copolymer grafted with at least one functional (anhydride) moiety at a secondary or tertiary carbon position via a free radical initiated reaction.
Silicon containing functional groups have also been grafted to the diene blocks of block copolymers as described in U.S. patent application Ser. No. 136,622, filed Dec. 22, 1987, now U.S. Pat. No. 4,822,857. Specifically, silicon compounds having at least one vinyl group are grafted to the diene blocks with some cleaving of the diene blocks using free radical extruder grafting.
However, such modified block copolymers do not take advantage of the arene block domain phenomena. Furthermore, the elastomeric properties of the polymer may be adversely altered by modifying or functionalizing the polymer B block. Thus, focusing on improving the high temperature capabilities of the block copolymer, it is necessary that the functional groups be grafted primarily in the arene block, A, such as is disclosed in copending U.S. patent applications Ser. Nos. 766,217 and 079,380, now U.S. Pat. No. 4,868,245.